1. Field of the Invention
The present invention relates to a liquid crystal dispaly apparatus and more particularly to a matrix liquid crystal display apparatus suitable for an image projecting display system.
2. Description of the Related Art
The related art will be explained with reference to FIGS. 1 and 2.
A liquid crystal material modulates orientation of molecules parallel to each other in large clusters, and has properties of a liquid as well. By applying a voltage to it, the orderly arrangement of the molecules is changed accordingly, and results to change the material's optical characteristics. The one of the applications of these characteristics is known as a liquid crystal display.
The liquid crystal display consists of two kinds of electrodes, one being a common electrode, the other being pixel electrodes that are installed opposing to the common electrode, with a liquid crystal material interposed therebetween. A data signal that is applied to the pixel electrode controls the optical characteristic of the liquid crystal material.
Liquid crystal display device is generally categorized into a transmission display type and a reflex display type. Transmission display type has comparatively a simple optical system, and is easy to produce economically, but has a demerit that the smaller a panel size of transmission display type is, the more increases the area occupation factor of switching transistors that selectively drive pixels of the display, and of electric wirings. As a result, the aperture factor falls down, and the display decreases its brightness.
On the contrary, the reflex display type, as known from the Japanese Patent Publication S57-39422/1982, the Japanese Patent Laid-Open Publication H4-338721/1992 and the U.S. Pat. No. 5,056,895, has a large aperture factor on a small display panel because switching transistors and electric wirings are placed behind the refractive segment electrodes. Accordingly, a reflex display type that is small in its size and packed densely, is suited for a projection type liquid crystal display system.
FIG. 1 shows a schematic diagram of a unit of a reflex liquid crystal display device of prior art which uses a metal oxide semiconductor field effect transistor (MOSFET). Switching element 1 is a MOSFET of which source or drain is connected to both a pixel electrode 2 and a storage capacitor 3. A liquid crystal layer 5 separates the common electrode 4 and the pixel electrode 2 each other. A gate electrode 6 is connected to the gate line Xi which carries control signals. A source electrode or a drain electrode which is not bound to the pixel electrode 2 is connected to the signal line Yj which carries image signals.
This device operates as follows, for example, when the control signal is supplied to a gate electrode 6 through the gate line Xi, a switching element 1 (MOSFET) turns on, and the image signal fed through the signal line Yj passes through the switching element 1 and charges up the storage capacitor 3 and is simultaneously applied to the pixel electrode 2. When the control signal through the gate line Xi becomes down to zero, the charge stored in the storage capacitor 3 will maintain the voltages of the pixel electrode 2.
A liquid crystal 5 is supplied with a differential voltage of the pixel electrode 2 and the common electrode 4. This differential voltage controls the optical transmission coefficient of the liquid crystal 5. Accordingly, by controlling the differential voltages, electric signals are converted to the modulated light.
When a polarized light S is projected in the direction toward the pixel electrode 2 through the common electrode 4, this polarized light S is reflected by an optical reflector to be explained later to become a P-polarization. This reflected light passes through the liquid crystal 5 and the common electrode 4 again. The optical signal is modulated by the light passing through the liquid crystal 5.
An image is formed by arranging such units of pixels in matrix, and scanning the pixels in horizontal and vertical directions. The scanning method is, for example, that the switching elements being along the gate line Xi, are turned on, image signals charging up each capacitors of the pixels, then scanning them in the Y direction.
FIG. 2 shows a sectional view of a unit of pixels which constitutes the integrated matrix image device.
The switching element which is a MOSFET, includes the gate electrode 6, a drain 7, and a source 8. The gate electrode 6 is formed by poly-crystal silicon for example, on a gate insulator 9, and is connected to the gate line Xi, as shown in FIG. 1. The drain 7 is connected to the signal line Yj, as shown in FIG. 1.
The storage capacitor 3 for charging image signals is made by depositing a insulator film 12 such as silicon dioxide inserted between single crystal silicon substrate 10 and a capacitor electrode 11, which capacitor electrode 11 is connected to the source 8. The pixel electrode 2 is formed on a silicon dioxide insulating layer 13, over the storage capacitor 3 and the switching element 1 each other. The pixel electrode 2 is connected to the source 8 electrically through an opening hole 14, and is made on the insulating layer 13 being silicon dioxide for example.
Moreover, an optical reflector 15 which is made by depositing at least two layers of insulating film such as silicon dioxide is deposited on a base structure which includes the substrate 10 and the pixel electrode 2. A first orientation film 16 is also deposited on the optical reflector 15.
A transparent glass 17 opposes to the silicon substrate 10 which includes the base structure, the optical reflector 15, and the first orientation film 16. A transparent common electrode 4 is deposited on the transparent glass 17, and a second orientation film 22 is further deposited on the common electrode 4. The liquid crystal display device is made by enclosing the liquid crystal 5 with the common electrode 4 and the optical reflector 15 which is a part of the base structure.
An incident light 18 which comes from above the transparent glass 17, passes through the liquid crystal 5. After being reflected by the optical reflector 15, the incident light 18 changes to a modulated light 19 and comes out from the liquid crystal image panel.
Upon the gate signal turns off, the storage capacitor 3 becomes an open circuit, and the electric charge therein is discharged slowly through a cutoff resistance of the switching element 1 and a resistance component of the liquid crystal 5, although, this electric leakage is not a problem to the operation of the liquid crystal display device, because the amount of the leakage is so small.
The problem is that the incident light 18 reaches to the switching element 1, penetrating through the optical reflector 15. Most of the incident light 18 is reflected by the optical reflector 15, but a part of the incident light 18 passes through the optical reflector 15 and irradiates the switching element 1 as a leakage light 18A which occurs in the area between pixel electrodes.
When the leakage light 18A irradiates the switching element 1, the switching element 1 turns to conductive state due to photo carriers generated in response to the leakage light 18A. In the case of that the silicon substrate 10 is p-type semiconductor, and the source 8 is n-type, among the photo carriers activated holes flow into the substrate 10, but are harmless. On the contrary, activated electrons flow into the source 3, which depletes the electric charge of the storage capacitor 8 causing the image panel to lose its image signals.
To improve this problem, it is conceivable to form the optical blocking layer between the pixel electrode 2 and the optical reflector 15. But this may cause following two problems, thus is not quite desirable.
One is an increase of required drive voltage because of the added impedance of the optical blocking layer, and another is a decrease of image resolution because of the widened electric field which is caused by the added optical blocking layer.